1. Field of the Invention
The invention relates to an automatic transmission for a vehicle. More specifically, it relates to the arrangement of the each transmission structural element in the gear train.
2. Description of the Related Art
An automatic transmission for a vehicle is required to multiply the gear ratios to improve driving performance and fuel consumption. To achieve the requirement, transmission mechanisms tend to go from a mechanism having four forward speeds to a mechanism having five forward speeds. For achieving further multiplication of the gear stages in the limited mounting space in a vehicle, a reduction in the number of elements of the gear train and a simplification of the mechanism are needed. A gear train, which establishes six forward speeds and one reverse speed, with a planetary gear set having minimum shift elements, is suggested in U.S. Pat. No. 5,106,352. In the gear train, forward six speeds are established by using an input rotation and outputting two decelerated rotations from the input rotation that are inputted to a planetary gear set having four shift elements.
In the aforementioned gear train, the number of shift elements and the number of clutches and brakes needed to establish the gear stages are reasonable. However, the gear train has problems which have to be improved. In the gear train, two clutches, which input the decelerated rotation transmitted through the reduction planetary gear to two different elements of the planetary gear set, are needed. These clutches need larger torque capacities than a clutch receiving the usual non-decelerated rotation because the clutches transmit the torque amplified by the deceleration. In the aforementioned structure, two clutches, which transmit the decelerated rotations, are arranged in one side of the reduction planetary gear in which the sun gear is always fixed and acts as a reaction element. Further, one clutch of the two clutches is arranged apart from the reduction planetary gear. Therefore, a long member for transmitting a high torque is needed. As a result, the size and the weight of the transmission are increased.